Morphometric and physiological development confirm skull ossification level as a reliable indicator of age in a passerine bird, the Noisy Miner

Abstract

<p>In birds, several physiological characters have been proposed and tested as possible criteria of age. Among them, skull development is likely to provide a valuable reflection of other physiological developments, however the relationship between skull ossification and other physiological and morphometric traits has rarely been quantified within species. Here, we investigated suitability of skull ossification as an indicator of age in Noisy Miners (<i>Manorina melanocephala</i>), an Australian passerine bird. We compared skull ossification with other age indicators including the bursa of Fabricius, gonadal development, and a range of commonly used body size measures that included wing length and tarsus length. We carried out morphological measurements on 1251 individual Noisy Miner carcasses that had been collected during a culling experiment in November, December and May 2015–2016 from north-eastern NSW, Australia. The probability of a bursa of Fabricius being present in an individual bird changed with skull ossification level, with a significant reduction in the size of the bursa of Fabricius correlating with the degree of skull ossification. In male Noisy Miners, birds that were at a later stage of skull ossification had significantly larger testis size than those with a lower stages of skull ossification. In females, ovary size increased significantly as skull ossification progressed. In both sexes, skull ossification level was also correlated with body mass and skeletal measures. Given this, the level of skull ossification in Noisy Miners accurately correlates with other age indicators, and can justifiably be used as a non-invasive aging method in the field for live birds.</p>